Linear power amplifiers are well known in the art. In general, these devices serve to provide voltage gain and/or current gain to a relatively low level input signal. Linear power amplifiers have had a wide variety of applications, such as servo-control, cathode ray tube deflection, audio frequency signal amplification, electric motor drivers, etc. Unfortunately, many prior art linear power amplifiers suffer from various deficiencies, including high power consumption and high heat generation.
Many prior art linear power amplifiers utilize a multi-stage design. First, the low level input signal (e.g. from a microphone amplifier, a CD player, a computer sound card, etc.) is fed into a voltage amplifier. The low level input signal experiences a voltage gain when it passes through the voltage amplifier. The voltage-amplified signal is then passed through a current amplifier. The signal experiences a current gain when it passes through the current amplifier. The voltage-amplified and current-amplified signal is then applied to a load such as a loudspeaker.
With prior art linear power amplifiers of this sort, it has been found that so long as the voltage amplitude of the signal being applied to the current amplifier remains below a certain level vis-a-vis the voltage powering the current amplifier, the current amplifier will not saturate and it will be able to power the load without degradation of the signal. However, when the voltage amplitude of the signal being applied to the current amplifier rises high enough relative to the level of the voltage powering the current amplifier, the current amplifier will saturate, thus clipping the signal voltage and limiting the current output to the load.
Typically, this problem is avoided by simply boosting the level of the voltage powering the current amplifier so as to eliminate clipping. Unfortunately, this solution has the effect of increasing the power consumed by the linear power amplifier and, consequently, increasing the heat generated by the various amplifier components. This in turn results in the need to apply additional heat sinks to the linear power amplifier, thus increasing size and cost.
Other prior art power amplifiers have been proposed to minimize the foregoing problems associated with linear power amplification. For example, in U.S. Pat. No. 4,042,890 issued Aug. 16, 1977 to Eckerle, a power amplifier is disclosed which uses a signal processor to control the state of a controllable switch, which in turn gates the output of a source of relatively high AC voltage. The Eckerle amplifier can be used to drive a wide variety of loads. However, this type of circuit also requires intermediate analog-to-digital conversion of the original signal, thus adding to complexity.